Apparatus for linear transmitter with improved loop gain stabilization

ABSTRACT

A device for linear transmitter  150  with improved loop gain stabilization. The linear transmitter  150  includes a main amplifier loop  178  and an auxiliary loop  176.  The device includes an adjustable driver  159,  connected within the main amplifier feedback loop  178  and further connected to the auxiliary loop  176.  The adjustable driver  159  receives an input signal from main amplifier feedback loop  178,  amplifies it in accordance with a gain control signal received from the auxiliary loop  176  and provides the amplified signal to main amplifier feedback loop  178.

FIELD OF THE INVENTION

[0001] The present invention relates to radio frequency transmitters ingeneral, and to linear radio frequency transmitters having a varyingantenna load, in particular.

BACKGROUND OF THE INVENTION

[0002] Radio communication devices use antennas to provide efficienttransmission of radio frequency communication signals. The transmissionportion of a communication device includes a power amplifier foramplifying the radio frequency signals before they are coupled to theantenna for transmission. The power amplifier design often relies onconstant load impedance which is directed at maximizing gain,efficiency, power output level, and the like. The behavior of atransmitter may be affected by its operating environment. For example, atransmitter operating near an electromagnetically reflective structuremay be susceptible to energy reflected back through the antenna into thetransmitter. Reflective energy may be detrimental to transmitterperformance, particularly to the performance of the power amplifier. Anisolator or circulator is often inserted between the antenna and thepower amplifier to protect against changes in load impedance as a resultof reflected energy.

[0003] The isolator protects the power amplifier by absorbing thereflected energy and preventing it from reaching the amplifier. Theisolator directs the reflected energy to an absorptive load termination.An isolator typically adds significant cost, size and weight to thedesign of a radio communication device.

[0004] U.S. Pat. No. 5,564,087 to Cygan et al., entitled “Method andapparatus for a linear transmitter” discloses another solution to theproblem of reflected energy. The solution incorporates a directionalcoupler to detect the reflected energy and provides a means of adjustingthe gain of the power amplifier accordingly. Generally, the gain to thepower amplifier is reduced when high levels of reflected energy arepresent. In this approach, the circuitry for detection of the reflectedenergy must operate at the transmission frequency. This adds significantcost and complexity to the radio design.

[0005] U.S. Pat. No. 5,675,286 to Baker et al., entitled “Method andapparatus for an improved linear transmission”, is directed to a methodand apparatus for isolator elimination, operative at the basebandfrequencies, which is described in detail, herein below. Reference ismade to FIG. 1, which is a schematic illustration of a lineartransmitter block of a radio communication device, generally referenced50, which is known in the art. Transmitter block 50 includes anattenuator 52, three summators 54, 66 and 70, a baseband loop filterunit 56, an up-mixer and radio-frequency (RF) filter unit 58, an RFpower amplifier 60, a down-mixer and baseband filter unit 64, a phaseshifter unit 62, an AGC 68, an adaptor unit 72 and an antenna 74.Summator 54 is connected to attenuator 52, to baseband filter unit 56and to phase shifter unit 62. Summator 66 is connected to basebandfilter unit 56, to up-mixer and RF filter unit 58, to adaptor unit 72and to AGC 68. Summator 70 is connected to AGC 68, to attenuator 52 andto adaptor unit 72. Adaptor 72 is further connected to AGC 68, toattenuator 52 and to phase shifter unit 62. Up-mixer and RF filter unit58 is connected to baseband loop filter unit 56 and to power amplifier60. Down-mixer and baseband filter unit 64 is further connected to poweramplifier 60 and to phase shifter unit 62. Antenna 74 is connected topower amplifier 60 and to down-mixer and baseband filter unit 64.

[0006] A signal 80 is provided as input to amplifier feedback loop 78and to isolator elimination circuit 76. Amplifier feedback loop 78 andisolator elimination circuit 76 represent the main amplification loopand the auxiliary loop, respectively. Amplifier feedback loop 78 is aclosed loop amplifier structure. Typically, this structure can beconsidered a Cartesian feedback loop amplifier. The input signal 80 isgenerally a complex digital baseband signal, having quadraturecomponents, i.e. in-phase (I) component and quadrature (Q) component.Signal 80 is provided to attenuator 52. Attenuator 52 provides anattenuated signal to summator 54. Summator 54 combines this signal witha feedback loop output signal 82 and provides a resulting error signalto baseband filter unit 56. Baseband filter unit 56 provides thefiltered error signal to up-mixer and RF filter unit 58. Up-mixer and RFfilter unit 58 up-converts the signal to RF and provides it to poweramplifier 60. Power amplifier 60 amplifies the signal and provides theamplified signal to antenna 74 for transmission. Antenna 74 forms a loadfor power amplifier 60. It is noted that this load is susceptible toimpedance variations due to its operating environment. Power amplifier60 provides a portion of the output signal to summator 54, viadown-mixer and baseband unit 64 and phase shifter unit 62, therebygenerating feedback loop output signal 82. Feedback loop output signal82 constitutes a feedback signal for controlling the gain of poweramplifier 60 and maintaining transmitter block 50 in the linear mode ofoperation.

[0007] Baseband filter unit 56 also provides a filtered error signal 90to summator 66. Summator 66 combines error signal 90 with signal 84 fromadaptor 72 and provides the result to AGC 68. AGC 68 constitutes alinear gain control circuit of isolator elimination circuit 76. Adaptor72 controls the gain of AGC 68 by altering the output signal of AGC 68.AGC 68 provides the output signal to summator 70, where it is combinedwith input signal 80. Summator 70 provides the resulting error signal 94to adaptor 72, which produces two output control signals 86 and 88.Control signal 86 adjusts the gain of attenuator 52, and control signal88 adjusts phase shifter 62. Adaptor 72 produces control signals 84, 86and 88 based on input signal 80 and error signal 94.

[0008] In order to optimise the function of the design above, it isnecessary to design a transmitter with sufficient gain and/or powerreserve. Such a design will result in increased complexity, cost, powerconsumption and more There is a trade-off between the desired designsimplicity and cost-effectiveness on one hand, and necessary dynamicrange of the system gain and/or output power, on the other hand. It isdesirable to provide a linear transmitter, which is cost-effective andsimple, yet assuring linear performance and high signal-to-noise ratio.

[0009] U.S. Pat. No. 5,542,096 to Cygan et al. is entitled ‘Method for aTransmitter to Compensate For Varying Loading Without An Isolator’. InU.S. Pat. No. 5,542,096, FIG. 1 shows a linear transmitter. In thetransmitter, a variable gain stage is located between a summing junction102, and a mixer 120.

SUMMARY OF THE INVENTION

[0010] It is an object of the present invention to provide a novelmethod and device for a linear transmitter with improved loop gainstabilization, which alleviates the disadvantages of the prior art.

[0011] In accordance with the present invention, there is thus provideda device for minimizing performance degradation of a linear transmitterblock of a radio communication system. The linear transmitter blockincludes a main amplifier feedback loop and an auxiliary loop. Thedevice includes an adjustable driver, connected within the mainamplifier feedback loop and further connected to the auxiliary loop.

[0012] The adjustable driver receives an input signal to be amplifiedfrom the main amplifier feedback loop and a gain control signal from theauxiliary loop. The adjustable driver amplifies received input signal inaccordance with the value of the gain control signal and provides theamplified signal to the main amplifier feedback loop.

[0013] In accordance with a further aspect of the present invention, themain amplifier feedback loop can include an attenuator, a basebandfilter, an up-mixer and RF filter unit, an RF power amplifier, a downmixer and baseband filter unit, a phase shifter and a summator. Thesummator is connected to the attenuator, to the up-mixer and RF filterunit and to the phase shifter unit. The adjustable driver is connectedto up-mixer and RF filter unit and to the RF power amplifier. Thedown-mixer and baseband filter unit is connected to the RF poweramplifier and to the phase shifter unit.

[0014] In accordance with yet a further aspect of the present invention,the auxiliary loop includes an adaptor, an AGC and two summators. Thefirst summator is connected to the baseband filter unit, to the adaptorand to the AGC. The second summator is connected to the attenuator, tothe AGC and to the adaptor. The adaptor is further connected to theattenuator, to the phase shifter unit, to the AGC and to the adjustabledriver.

[0015] The input signal to be amplified constitutes a first basebanderror signal, filtered by the baseband filter and up-converted by theup-mixer and RF filter unit. The first baseband error signal is providedby the summator and constitutes a sum of a baseband signal and an outputsignal of the main amplifier feedback loop. The baseband signal is aninput baseband signal attenuated by the attenuator. The attenuation gainof the attenuator is controlled by a first output signal from theadaptor. The output signal of the main amplifier feedback loop is aresult of down converting by the down-mixer and baseband filter unit,and phase shifting by the phase shifter of a portion of an output RFsignal provided by the RF power amplifier. The phase shifter iscontrolled by a second output signal from the adaptor.

[0016] In accordance with a further aspect of the present invention, theadaptor provides to the adjustable driver the gain control signal. Thisgain control signal is a result of a comparison between the inputbaseband signal and a second baseband error signal, provided by thesecond summator of the auxiliary loop. The second baseband signal is asum of the input baseband signal and an output signal from the AGC. Theoutput signal from the AGC is a result of a comparison between the gaincontrol signal and a third baseband error signal provided by the firstsummator of the auxiliary loop. The third baseband error signal is a sumof a third output signal from the adaptor and the first baseband errorsignal, provided by the baseband filter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The present invention will be understood and appreciated morefully from the following detailed description taken in conjunction withthe drawings in which:

[0018]FIG. 1 is a schematic illustration of a linear transmitter blockof a radio communication device, which is known in the art; and

[0019]FIG. 2 is a schematic illustration of a linear transmitter blockof a radio communication device, constructed and operative in accordancewith a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0020] The present invention overcomes the disadvantages of the priorart by providing an adjustable driver for improving loop gainstabilization.

[0021] Reference is now made to FIG. 2, which is a schematicillustration of a linear transmitter block of a radio communicationdevice, generally referenced 150, constructed and operative inaccordance with a preferred embodiment of the present invention.Transmitter block 150 includes an attenuator 152, three summators 154,166 and 170, a baseband loop filter unit 156, an up-mixer andradio-frequency (RF) filter unit 158, an adjustable driver unit 159, aRF power amplifier 160, a down-mixer and baseband filter unit 164, aphase shifter unit 162, an AGC 168, an adaptor unit 172 and an antenna174. Summator 154 is connected to attenuator 152, to baseband filterunit 156 and to phase shifter unit 162. Summator 166 is connected tobaseband filter unit 156, to up-mixer and RF filter unit 158, to adaptorunit 172 and to AGC 168. Summator 170 is connected to AGC 168, toattenuator 152 and to adaptor unit 172. Adaptor 172 is further connectedto AGC 168, to adjustable driver 159, to attenuator 152 and to phaseshifter unit 162. Up-mixer and RF filter unit 158 is connected tobaseband loop filter unit 156 and to adjustable driver 159. Poweramplifier 160 is connected to adjustable driver 159 and to antenna 174.Down-mixer and baseband filter unit 164 is connected to power amplifier160 and to phase shifter unit 162. Antenna 174 is connected to poweramplifier 160 and to down-mixer and baseband filter unit 164.

[0022] A signal 180 is the input signal to an amplifier feedback loop178 and an isolator elimination circuit 176. Amplifier feedback loop 178and isolator elimination circuit 176 represent the main amplificationloop and the auxiliary loop, respectively. Amplifier feedback loop 178is a closed loop amplifier structure. Typically, this structure can beconsidered a Cartesian feedback loop amplifier. The input signal 180 isgenerally a complex digital baseband signal having quadraturecomponents, i.e. in-phase (l) component and quadrature (Q) component.Signal 180 is provided to attenuator 152. Attenuator 152 provides anattenuated signal to summator 154. Summator 154 combines this signalwith a feedback loop output signal 182 and provides a resulting errorsignal to baseband filter unit 156. Baseband filter unit 156 providesthe filtered error signal to up-mixer and RF filter unit 158. Up-mixerand RF filter unit 158 up-converts the signal to RF and provides it toadjustable driver 159. Adaptor 172 controls the gain of adjustabledriver 159. Adjustable driver 159 provides the output signal to poweramplifier 160. Power amplifier 160 amplifies the signal and provides theamplified signal to antenna 174 for transmission. Antenna 174 forms aload for power amplifier 160. It is noted that this load is susceptibleto impedance variations due to its operating environment. Poweramplifier 160 provides a portion of the output signal to summator 154,via down-mixer and baseband unit 164 and phase shifter unit 162, therebygenerating feedback loop output signal 182. Feedback loop output signal182 constitutes a feedback signal for controlling the gain of poweramplifier 160 and maintaining transmitter block 150 in the linear modeof operation.

[0023] Baseband filter unit 156 also provides a filtered error signal190 to summator 166. Summator 166 combines error signal 190 with signal184 from adaptor 172 and provides the result to AGC 168. AGC 168constitutes a linear gain control circuit of isolator eliminationcircuit 176. Adaptor 172 controls the gain of AGC 168 by altering theoutput signal of AGC 168. Adaptor 172 controls also the gain ofadjustable driver 159. AGC 168 provides the output signal to summator170, where it is combined with input signal 180. Summator 170 providesthe resulting error signal 194 to adaptor 172, which produces two outputcontrol signals 186 and 188. Control signal 186 adjusts the gain ofattenuator 152, and control signal 188 adjusts phase shifter 162.Adaptor 172 produces control signals 184, 186 and 188 based on inputsignal 180 and error signal 194.

[0024] In comparing the linear transmitter 150 of the present inventionwith that of the prior art, it is significant to note the presence ofadjustable driver 159. Its gain, which is dynamically controlled byadaptor 172, depends on information contained in error signals 190 and194. Since adjustable driver 159 adds additional gain to feedback loop178, it is possible to use low-power isolator elimination circuitry, yetmaintaining required loop gain and linearity of the system 150.

1. Device for minimizing performance degradation of a linear transmitterblock (150) of a radio communication device in the presence of antennainterference, the linear transmitter block including a main amplifierfeedback loop (178) and an auxiliary loop (176), the device comprisingan adjustable driver (159), connected within said main amplifierfeedback loop (178) and further connected to said auxiliary loop (176),said adjustable driver (159) receiving an input signal to be amplifiedfrom said main amplifier feedback loop (178), said adjustable driver(159) receiving a gain control signal from said auxiliary loop (176),said adjustable driver (159) amplifying said received input signal,according to said gain control signal, and said adjustable driver (159)providing said amplified signal to said main amplifier feedback loop(178).
 2. The device according to claim 1, wherein said main amplifierloop (178) includes an attenuator (152), a baseband filter (156), anup-mixer and RF filter unit (158), a RF power amplifier (160), adown-mixer and baseband filter unit (164), a phase shifter (162) and asummator (154), wherein said summator (154) is connected to saidattenuator (152), to said base band filter (156) and to said phaseshifter unit (162), wherein said adjustable driver (159) is connected tosaid up-mixer and RF filter unit (158) and to said RF power amplifier(160), said down-mixer and baseband filter unit (164) is connected tosaid RF power amplifier (160) and to said phase shifter unit (162);wherein said auxiliary loop (176) includes an adaptor (172), an AGC(168) and two summators (166) and (170), said summator (166) isconnected to said baseband filter unit (156), to said adaptor (172) andto said AGC (168), said summator (170) is connected to said attenuator(152), to said AGC (168) and to said adaptor (172), and said adaptor(172) is connected to said AGC (168), to said attenuator (152), to saidphase shifter unit (162) and to said adjustable driver (159).
 3. Thedevice according to claim 2, wherein said input signal to be amplifiedconstitutes a first baseband error signal, filtered by said basebandfilter (156) and up-converted by said up-mixer and RF filter unit (158),said first baseband error signal being provided by said summator (154),and constitutes a sum of a baseband signal and an output signal of saidmain amplifier feedback loop (178), said baseband signal being an inputbaseband signal attenuated by attenuator (152), said attenuatorcontrolled by a first output signal from said adaptor (172), and saidoutput signal of said main amplifier feedback loop (178) being a resultof down-converting by said down-mixer and baseband filter unit (164) andphase shifting by said phase shifter (162), of a portion of an output RFsignal provided by said RF power amplifier (160), wherein said phaseshifter being controlled by a second control signal from said adapter(172).
 4. The device according to claim 3, wherein said gain controlsignal is provided by said adaptor (172), said gain control signal is aresult of a comparison between said input baseband signal and a secondbaseband error signal, provided by said summator (170), said secondbaseband error signal is a sum of said input baseband signal and anoutput signal from said AGC (168), said output signal is a result of acomparison between said gain control signal and a third baseband errorsignal provided by said summator (166), and said third baseband errorsignal is a sum of a third output signal from said adapter (172) andsaid first baseband error signal, provided by said baseband filter(156).